At least 21 people were killed, about 60 injured and many others feared trapped after a flyover under construction collapsed in the eastern Indian city of Kolkata Thursday, officials said.

"12 dead bodies have been handed over to their families & 3 are awaiting final documentation. 6 bodies are still unidentified," according to AITC, the ruling party in the state of West Bengal, of which Kolkata is the capital.

About 80 meters of the flyover collapsed mid-morning in a congested part of the central Burrabazar district, trapping vehicles and people underneath it.

"There was a deafening noise and there was dust all around, it went down in seconds," Mahesh Sarkar, a witness to the collapse, said.

Rescue operations continued into the night, with the army, police and disaster relief force cutting and sifting through mangled metal and concrete debris to find those still trapped.

"One girder seems to have fallen and then another collapsed," said KP Rao, spokesman for IVRCL, the construction company responsible for the flyover. "We are not sure exactly what the problem was."

"The guilty will not be spared. We will take action against builders. This is a very serious incident," said Mamata Banerjee, chief minister of West Bengal state.

"But our first priority is to rescue the trapped, treat the injured in hospital and console the families of those who have died," Banerjee said.

Six units of the National Disaster Response Force were rushed to the site and cranes were being used to lift debris. News footage showed mangled, half-burnt cars being lifted out from the rubble.

Many people had head and neck injuries, and several suffered burns when their vehicles caught fire, police said.

"At least two of our engineers are missing," Rao said from IVRCL headquarters in Hyderabad. "We must have had 30 to 40 people working on the site," he said in remarks carried by several media.

The army has also been called in to help, said Banerjee, who called off an election rally to visit the site in the state capital. Around 400 army personnel including engineers and medical personnel were deployed, reports said.

Residents formed a human chain to keep onlookers back from rescue operations, reports said.

"There are heavy concrete blocks. The operation will take some time," said Kiren Rijiju, federal junior minister for home affairs.

"Shocked and saddened by collapse of under construction flyover in Kolkata," Primer Minister Narendra Modi posted on Twitter.

Construction on the flyover started in 2008 but had stopped and started several times amid apparent financial trouble suffered by IVCRL, according to an India Today report.

The flyover was almost the width of the road, and passed close to rows of three- and four-storey houses and shops on both sides, in an extremely congested area in the north of the city.

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